Add/use new text.dat strings for received Fido and Internet mail

- InternetMailReceived
- InternetMailForwarded
- FidoNetMailReceived
- WithAttachment
- FidoEchoMailReceived

For The Millionaire, closing issue #254

The first string (suppressed with %.0s) is an optional date/time stamp.

This also deprecates (removes support for) the [mail] NewMailNotice and ForwardNotice keys from sbbs.ini.